Chemical Engineering December 2011 - 33

Dickow Pumps, USA
Teikoku Pumps, USA
FIGURE 6. This canned motor
pump is fitted with a high-tem
emperature
motor winding, which
allows operation up to 650°F
(343°C) without the use of cool
ing water. The pump may also be
en(343°C)
without the use of cooloperated
as cold as -40°C (-40°F) or
colder. The electrical junction box is
located at a distance from the hot
FIGURE 5. This cutaway of a magnetic drive pump
shows the outer magnet, isolation can and the interior
rotating assembly. The impeller and interior magnet
are supported by ceramic sleeve bearings - that
are wetted by the fluid - inside the pump. The outer
magnet is supported by the ball bearings seen on
the right. The containment shell prevents fluid from
escaping from the pump, but is permeable to the
magnetic field allowing the outer magnet to drive the
inner magnet and impeller assembly. The pump casing
is supported by pedestals connected to a centerline
mount to help maintain alignment. The magnetic
drive assembly is separated from the hot end of the
pump by an extended adapter with cooling fins. Properly
fitted, this pump can operate from -60 to 400 °C
-
rior rotating assembly. The impeller and interior mag
increase. The cooling heat
exchanger is normally a
liquid-to-liquid heat exchanger
where thermal
fluid is cooled by water,
although there are different
approaches. The
exchanger is generally
rated at a temperature
below the maximum temphase
motor winding is wrapped
around the can. When the winding
is energized, the pump operates like
an electric motor. As with magnetic
drive pumps, the rotating-assembly
bearings are lubricated and cooled
by the pumped fluid.
Sealless pumps (both canned-motor
and magnetic-drive) have the advantage
that they do not leak, but the
user should be mindful of certain other
aspects of sealless pump use. Sealless
pumps use special materials of
construction and have a significantly
higher first cost than conventional
pumps with mechanical seals. However,
if they are properly specified, installed
and operated, sealless pumps
can enjoy a long service life that helps
to offset their initial high cost.
Sealless pumps are more sensitive
to suction conditions than their mechanically
sealed cousins. They do not
tolerate cavitation, slugging or dry
running, and can fail quite dramatically
if subjected to a significant diet of
these conditions. Repair costs for failed
sealless pumps can be quite costly. For
these reasons, an installation utilizing
sealless pumps should be carefully
evaluated to avoid instances of cavitation,
slugging or dry running.
Cooling protection
Cooling problems increase as the temperature
difference (∆T) between the
thermal fluid and the cooling media
perature anticipated in the system in
order to have an acceptable cooling
rate at lower process temperatures.
This presents a few interesting problems
when the thermal fluid is at
maximum temperature and the system
transitions over to cooling mode.
If the heat transfer fluid is hot
enough, and the ∆T is very high, the
heat exchanger will operate at a heat
transfer rate well above its specified
rating. If the temperature is high
enough, the cooling water can actually
boil on (or in) the tubes. Depending
on cooling water quality, this can
allow scaling to occur on the tubes and
eventually lower the efficiency of the
heat exchanger and require expensive
chemical cleaning. Another possibility,
again depending on water quality and
condition, is to have corrosion on the
tubes, which will eventually lead to a
tube leak.
When operating at high thermalfluids
temperatures,
and above
the specified rating of the heat exchanger,
it is also possible to overheat
the water returning to the tower. This
problem is particularly important
if the user goes to a dedicated cooling
tower, because the cooling-waterreturn
temperature can exceed the
maximum temperature rating of the
tower. If the tower has a plastic fill,
the plastic can actually soften and
sag in the tower. More than one tower
has been rebuilt as a result of high
cooling-water-return temperatures.
Chillers are sometimes used to directly
cool the thermal fluid, particularly
if cold temperatures are required
to finish the batch. Chillers are also
used to avoid maintaining a cooling
tower. Again, serious (and sometimes
expensive) problems can surface if
fluid supply temperature exceeds the
maximum allowable temperature of
the chiller. Problems can range from
reduced service life of the chiller to
out-and-out catastrophic failure.
To avoid these pitfalls, consideration
needs to be given to methodologies
that guard against extreme
temperatures. This can generally
be accomplished by the use of flowcontrol
and recirculation strategies,
along with careful specification of
heat exchangers, cooling towers and
chillers. The benefits realized will be
reduced maintenance and longer life
of critical equipment.
■
Edited by Dorothy Lozowski
Author's note:
Pictures included in the article are printed with
the permission of the manufacturer. Use of these
pictures does not constitute an endorsement
of the product. When specifying equipment for
thermal fluid systems, the reader should employ
experienced resources, either internal or external,
to insure that recognized and generally accepted
good engineering practice is followed and
that local and national codes and standards are
complied with.
Author
Jay Hudson is the principal
consultant of J. G. Hudson &
Associates (P.O. Box 4064,
Salisbury, N.C.
28145-4064;
Website:
Email:
www.projeng.com;
jhudson@projeng.
com; Phone: 704-637-1714), a
specialty firm concentrating
in thermal-fluid consulting
services. Hudson is a professional
engineer licensed in
the state of North Carolina
with 35 years of experience in the chemical
process industries (CPI) and concentrates his
efforts exclusively in the area of thermal-fluidsystem
design, specification and operation. He
was recently appointed to the NFPA Technical
Committee on Fluid Heaters (NFPA 87).
CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2011 33
pump to allow the use of conven
tional wiring to connect the pump.
The dial face on the junction box
monitors wear of the internal bearings
http://www.projeng.com http://WWW.CHE.COM
Chemical Engineering December 2011

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